An Obama administration official visited North Korea twice in 2012 in an unsuccessful effort to improve relations after Kim Jong Un assumed power, ex-officials say.

WASHINGTON — A White House official made two secret visits to North Korea last year in an unsuccessful effort to improve relations after new ruler Kim Jong Un assumed power, according to former U.S. officials familiar with the trips.

The brief visits in April and August were aimed at encouraging the new leadership to moderate its foreign policy after the death of Kim's father, longtime autocrat Kim Jong Il, in December 2011.

The ruling elite apparently spurned the outreach effort, however. This month, after a surge of fierce anti-U.S. rhetoric, the government in Pyongyang defied international warnings and conducted its third and most powerful underground nuclear test.

The former U.S. officials, who spoke on condition of anonymity because the back-channel trips have not been formally disclosed, said the first visit was an unsuccessful attempt to persuade Pyongyang not to launch a long-range rocket.

North Korea carried out the launch April 12. The missile flew only a few minutes before it exploded and crashed into the sea. A subsequent test of another long-range rocket in December was successful.

The April trip was led by Joseph DeTrani, a North Korea expert who then headed the National Counter Proliferation Center in the Office of the Director of National Intelligence, which coordinates U.S. intelligence agencies, the former U.S. officials said.It was unclear who led the August trip.

They said Sydney Seiler, who is in charge of Korea policy at the National Security Council, went on both trips. Seiler, a veteran CIA analyst, speaks fluent Korean. He could not be reached for comment.

The White House, State Department and CIA have refused to confirm or deny the 2012 trips, which occurred during the U.S. presidential election season.

"I'm not going to comment on this," Tommy Vietor, spokesman for the National Security Council, said in an email.

U.S. officials have visited North Korea on and off for years. They include Secretary of State Madeleine Albright, who led an official state visit in 2000. The last official U.S. visit was in 2009 when U.S. special envoy Stephen Bosworth sought to restart stalled six-party negotiations on North Korea's nuclear program. The talks have not resumed.

DeTrani left the government last year and now heads the Intelligence and National Security Alliance, an industry group.

"There are certain things I just won't talk about, and this is one subject I really feel it's not appropriate for me to comment on," DeTrani said in a telephone interview.

Without confirming the 2012 trips, DeTrani said it "makes eminent sense" for the United States to conduct talks with North Korean officials after Kim Jong Il's death. DeTrani said he and other U.S. experts initially saw signs that Kim Jong Un might behave less rigidly than his father, including putting moderate figures in key government positions.

Those hopes were quickly dashed, however.

In addition to the rocket launches and nuclear test, the new leader appointed a defense minister, Gen. Kim Kyok-Sik, who reportedly was responsible for the 2010 shelling of a South Korean island that killed four people, and the sinking of a South Korean naval ship that killed 46 sailors.

"I was initially guardedly optimistic that [Kim Jong Un] was moving in the right direction," DeTrani said. "With the launches and the test, he's reversed that."

News of the secret trips first leaked out in the South Korean press, which said the flights from Guam to the Sunan airport in Pyongyang, North Korea's capital, passed through South Korean airspace.

"The South Korean air force was tracking the plane. They knew there was a special flight going to Pyongyang, but the purpose of the mission was a secret," said Moon Chung-in, a former South Korean government advisor on North Korea and a professor at Yonsei University in Seoul.

Some North Korea specialists applauded the Obama administration for breaking a long-standing taboo against one-on-one talks with Pyongyang.

"The trips were a good idea and I think the fact that they did them secretly was a good idea," said John Delury, a North Korea expert at Yonsei University. He visited Pyongyang in January on a private trip with Google's chairman, Eric Schmidt.

"I don't know why at this point the administration just doesn't set the record straight on this," said a former U.S. official. "All it shows is that we were trying to walk the last mile with North Korea."

Another analyst said disclosing the trips would subject them to scrutiny from Japan and South Korea, as well as Congress. The congressional intelligence and foreign affairs committees have not been briefed on the visits, officials said, although they would not necessarily be notified.

Chuck Downs, a former Pentagon official and Korea expert, characterized the visits as a worthy gambit.

"The history of this past year of seemingly unsuccessful efforts to seek common ground on which to deal with Kim Jong Un will, I suspect, be a very important study in foreign relations for years to come," he said in an email.

Japan's Asahi Shimbun newspaper, which also reported details of the trips, said U.S. military officials conducted an unannounced visit to Pyongyang in November 2011. It said officials of the U.S. Pacific Command discussed efforts to recover the remains of U.S. soldiers killed in the Korean War, which ended in 1953. The Times could not confirm that visit.